Conventionally, a compressor, which comprises a compressing mechanism for compressing an operating gas in a casing and an electric motor for being coupled with and driving the compressing mechanism, has been well known in general, and, for example, such compressor has been used for a refrigeration cycle, such as an air conditioner and the like. As an example or an electric motor for this kind of compressor, a permanent magnet-type electric motor, which comprises a stator for generating rotating magnetic field by turning on electricity to coils of stator yoke (core) made from magnetic material and a rotor which is disposed in the stator so as to rotate therein and in which a plurality of permanent-magnets forming a magnetic pole are inserted in magnet insertion portions of a rotor yoke (core) in such manner that the magnets form in line in the circumferential direction thereof, has been used.
As a permanent magnet-type electric motor like the above, it has been conventionally proposed, as shown in Japanese Laid-Open Patent Publication No. 2000-92763, that each magnet includes portions that have different magnetic forces in the width direction thereof (in the circumferential direction of yoke), wherein end portions of the magnet have smaller magnetic forces than a central portion of the magnet in order to prevent deterioration of magnetic flux density due to leakage flux between the permanent magnets. According to this structure, the leakage flux that may leak out by way of the yoke between adjacent magnets can be made small, and a distribution of magnetic flux in air gap between the rotor and the stator can be changed to be similar to substantially sine waves, resulting in improvement of high efficiency and high torque of the electric motor.
In the meantime, such rotor having permanent magnets inserted in the insertion portions of yoke has problems that the rotor may be heated under a high temperature condition where the electric motor operates and, if reverse magnetic field is caused by stator current to produce a desired torque of electric motor, each magnet may be demagnetized by these heating and reverse magnetic field.
In order to prevent this demagnetization of magnet, a magnet having large intrinsic coercive force (force of retaining magnetism) or a large thickness in the magnetic field direction may be applied. In this case, however, such magnet having large intrinsic coercive force tends to have smaller magnetic flux density, and deterioration of the max torque and the efficiency of electric motor is inevitable due to this small magnetic flux density.
On the other hand, in the event that a magnet with a large thickness in the magnetic field direction is used, much magnetic material is necessary for the increase of thickness, resulting in an increase in cost.
The present invention has been made in view of the above-described problems, and an object of the present invention is to prevent demagnetization of magnets as a whole without any cost increase and improve the max torque and the efficiency of electric motor by suppressing deterioration of magnetic flux density, by making improvements on a characteristic of each magnet inserted in the rotor yoke.